1. Field of the Invention
This invention relates to a catalyst for denitration by catalytic reduction using ammonia and a process for producing the catalyst. More particularly, it relates to a denitration catalyst suitable for denitrating exhaust gases containing a large quantity of heavy metal oxdes.
2. Description of the Related Art
Nitrogen oxides (NOx) exhausted from various fixed NOx - generating sources are a main air pollutant together with sulfur oxides (SOx). As to the process for removing this NOx, there are various processes, and among these, a process of selectively reducing NOx with ammonia added to exhaust gases in the presence of a catalyst has been constituting the main current of the processes. It is required for this catalyst for denitration by catalytic reduction using ammonia to be not degenerated by SOx and ashes contained in exhaust gases generated by combustion of fossil fuel such as petroleum, coal, etc., and for satisfying this requirement, various catalysts based on titanium oxide have been invented and now broadly practically used (Japanese patent application liquid-open Nos. Sho 50-128681/1975, Sho 53-28148/1978, etc.).
These catalysts are prepared by adding oxides of transition metal elements such as vanadium, molybdenum, tungsten, iron, chromium, etc. to metatitanic acid or titanium oxide by means of kneading, impregnation or the like, followed by calcination, and have superior activity and life to usual catalysts for denitration of exhaust gases from a combustion apparatus of petroleum, coal, etc.
However, in the case where exhaust gases containing a large quantity of vapors of heavy metal oxides such as combustion exhaust gases from low quality coal, exhaust gases from boilers having an ash-circulating line, etc. are treated, a problem of reduction in the catalyst activity has not been taken into consideration. In particular, in the case of a combustion system having an ash-circulating line 9 as shown in FIG. 5, metals contained in mineral substances of coal such as lead (Pb), selenium (Se), arsenic (As), cadmium (Cd), zinc (Zn), etc. are transferred into exhaust gases in the form of vapor of single substances or oxides during the process whrerein ashes are molten in a furnace 1 and recovered in the form of slug, and are present in the form of a high concentration of metal vapors upstream of an air preheater 6 usually provided with a denitration means (see H. Brumsack et al., Environmetal Technology Letters; 5, 7-22 (1984)), and it has raised a problem that the denitration catalyst is poisoned by these vapors (see Shokubai, 29, 36-37 (1987)).
Further, the above-mentioned prior art has given no adequate consideration to the catalyst strength, particularly to the hardwearing strength of the catalyst due to ash particles contained in exhaust gases.